1. Field of the Invention
The present invention relates to a hybrid vehicle having an internal combustion engine serving as a traveling source of the vehicle and also having another traveling source, and more particularly to control of avoiding knocking of an internal combustion engine in a case where an electricity storage mechanism as another traveling source has to be charged by operating a power generator by the internal combustion engine in idle off.
2. Description of the Background Art
A hybrid vehicle is put into practical use which includes an engine operated by combustion energy of fuel and a motor operated by electrical energy, as motive power sources during vehicle travel, and is provided with an automatic transmission between the motive sources and the drive wheels. In such a hybrid vehicle, a fuel consumption amount or a gas emission amount can be reduced with prescribed traveling performance being kept, for example, by separately using the engine and the motor for traveling depending on the driving conditions. Specifically, a plurality of driving modes with different operation conditions of an engine and a motor are provided, including an engine travel mode of traveling only using an engine as a motive power source, a motor travel mode of traveling only using a motor as a motive power source, an engine+motor travel mode of traveling using both an engine and a motor as motive power sources, and the like. The driving modes are automatically switched according to a predetermined mode switch condition such as a motive power source map using such driving conditions as a vehicle speed (or the rotational speed of the motive power source) and an acceleration operation amount as parameters.
In order to supply electricity to this motor, an electricity storage mechanism is mounted on such a hybrid vehicle. As an electricity storage mechanism, in general, a secondary battery is often mounted for such reasons as energy density and charging/discharging efficiency. As the kinds of this secondary battery, a high-voltage nickel metal hydride battery, a lithium ion battery, and the like are used. It is noted that a motor generator is used as a motor for traveling to perform a regenerative operation of energy at a time of braking, thereby improving a fuel consumption rate. Here, the electricity storage mechanism may be a capacitor.
On the other hand, in an engine including a spark plug, ignition timing control is performed in order to obtain an output obtained by combustion efficiently as much as possible and to obtain better emission cleaning performance and fuel efficiency performance. Here, in order to obtain energy produced by combustion as an output most efficiently, it is known that the pressure peak in the interior of a combustion chamber preferably occurs slightly behind the compression top dead center. Therefore, the ignition timing is determined such that the pressure peak occurs slightly behind the compression top dead center, and if the ignition timing is too early (advanced too much), knocking is caused.
The ignition timing at which an engine produces the maximum torque is called MBT (Minimum spark advance for Best Torque) and, MBT is in proximity of the ignition timing at which knocking starts occurring, though depending on the kind or the rotational speed of the engine. Then, a Knock Control System (KCS) is mounted so that the optimum output is obtained while knocking is suppressed. The Knock Control System repeats a gradual advance in a case where no knocking occurs, a gradual retard until knocking stops when knocking is detected, and a gradual advance again when knocking stops.
In other words, knocking is suppressed by performing retard correction on the ignition timing based on the correction amount increased and decreased depending on whether knocking occurs or not and thus suppressing a temperature increase of the combustion chamber. The reason why a temperature increase in the combustion chamber can be suppressed by such retard correction of the ignition timing is that the combustion period of fuel-air mixture in the combustion engine is shifted to the retard side by retarding the ignition timing, so that the fuel-air mixture is sent as emissions to the emission path with its high combustion temperature being kept, and the heat during combustion of the fuel-air mixture is less likely to be transferred to the combustion chamber. It is noted that the ignition timing at the limit at which knocking does not occur is called the knock limit ignition timing.
In the normal ignition timing control, the ignition timing is controlled using a basic ignition timing that is predetermined depending on the driving conditions, and a KCS correction amount which is the amount of correction from this basic ignition timing to the knock limit ignition timing. In other words, the control is performed with the ignition timing=the basic ignition timing+the KCS correction amount (this KCS correction amount is, for example, the basic advance amount+the corrective advance amount). Of course, the KCS correction amount may be the basic retard amount+the corrective retard amount.
Japanese Patent Laying-Open No. 2000-130204 discloses a control device which avoids an increase of the emission temperature by the retard correction of the ignition timing as measures against knocking of the engine in a hybrid vehicle and also compensates for reduction of the engine output at the time of knocking control. This control device controls a hybrid vehicle including an internal combustion engine and an electric rotating machine as motive power devices and including a variable valve device variably controlling the intake valve operation timing of the internal combustion engine depending on the driving conditions and a knocking detection device detecting occurrence of knocking of the internal combustion engine. This control device controls the intake valve operation timing by the variable valve device and the output of the electric rotating machine depending on the knocking occurrence state. This control device is configured to control the intake valve operation timing in the retard direction from the basic operation timing during occurrence of knocking and also compensate for the reduced output of the internal combustion engine due to this retard control using the electric rotating machine.
According to this control device of a hybrid vehicle, the intake valve operation timing (in particular, the valve closing timing) is controlled in the retard direction from the basic operation timing during occurrence of knocking. Accordingly, the intake air filling rate and the effective compression ratio are reduced, thereby eliminating knocking. The knocking intensity can also be alleviated by delaying either the ignition timing or the intake valve operation timing. On the other hand, while retarding the ignition timing increases the emission temperature, retarding the intake valve operation timing tends to reduce the emission temperature. Therefore, knocking can be eliminated by delaying the intake valve operation timing without increasing the emission temperature. Furthermore, in addition to retarding the intake valve operation timing in this manner, the electric rotating machine compensates for the reduced output of the internal combustion engine due to this retard control, which ensures the good driving performance while avoiding knocking.
Now, in the hybrid vehicle as described above, when the engine is operated, the electric storage mechanism is sometimes charged by operating the motor generator as a power generator with the accelerator off (idle on). In this case, the ignition timing control is performed by KCS such that the ignition timing is advanced in order to stabilize the idle state, using the aforementioned corrective advance amount. Therefore, in the idle state and in a minute-load operation state in which the motor generator is operated with the rotational force of the engine, the ignition timing is controlled toward the advance side by KCS, and in some cases, the KCS feedback value may stay on the upper limit guard value. In this case, if the driver of the hybrid vehicle heavily depresses the accelerator pedal (rapidly accelerates the vehicle), the load of the engine increases drastically, and it takes much time for the KCS feedback value to reduce (delay due to the feedback control), causing a delay before the finally calculated ignition timing is controlled toward the retard side. For example, the KCS feedback value is not reflected on the final ignition timing until it falls to the median value or smaller of the KCS feedback value, and knocking continuously occurs until then.
However, the aforementioned Japanese Patent Laying-Open No. 2000-130204 only copes with an increase of the emission temperature and an engine output reduction caused by the retard correction of the ignition timing for avoiding knocking and does not mention the knocking caused when the accelerator is turned on in the state in which the ignition timing is changed toward the advance side by KCS when the engine is under a minute load in the idle state.